1. Field of the Invention
The present invention relates to a wireless communication system and method thereof for inserting a guard interval into a symbol and transmitting the symbol to eliminate intersymbol interference due to multipath. In particular, the present invention relates to a wireless communication system and method thereof which use a plurality of guard interval lengths.
2. Description of the Related Art
For example, in a wireless communication system that implements OFDM (Orthogonal Frequency Division Multiplexing), a guard interval is commonly inserted to strengthen tolerance to multipath interference. Thus, as shown in FIG. 1, a guard interval is created by copying the signal at the tail-end part of an effective symbol and is inserted to the head and, with the effective symbol, serves as one OFDM symbol.
FIG. 2 shows the effect of delay waves that are generated due to multipath. When the longest delay due to multipath is shorter than the guard interval length, such as until delay wave 2 shown in FIG. 2, the adjacent symbol component does not enter during the FFT window period. Therefore, intersymbol interference due to multipath can be eliminated completely. However, if a delay wave 3, such as that shown in FIG. 3 exists, the adjacent symbol component enters and multipath interference is generated.
Therefore, increasing the guard interval length can reduce the effect of delay waves. At the same time, if the guard interval length is long, transmission efficiency is reduced, causing a reduction in bit rate.
Thus, although the guard interval length is preferably set to about the maximum path delay, the guard interval length must accommodate various cell placements and cell diameters when applying the OFDM wireless communication system to a cell phone system. It is impossible to determine the most suitable guard interval length for the overall system.
In addition, because the distribution of delay paths differ depending on the location of the terminal, even when the cell diameter is the same, it is difficult to configure the most suitable guard interval length.
As described above, it is difficult to apply one fixed guard interval length to all instances. Therefore, a wireless communication system is proposed, that can accommodate a plurality of guard interval lengths of long guard intervals and short guard intervals for each Sub-frame. The Sub-frame is the transmission unit of one transmission, as presented in Format #1 and Format #2 that are examples of data format into which the guard interval shown in FIG. 3 has been inserted, and performing adaptive control of the guard interval length through a wireless state is considered.
For example, in Patent Reference 1, below, a system, wherein a base station determines the guard interval length and notifies a mobile station of the guard interval length, is disclosed. In other words, as shown in FIG. 4 as a Conventional Example 1 related to the determination of guard interval length, this is a system wherein notification of guard interval information is given at every Sub-frame in a channel for notification (notification CH), in correspondence with a channel for transmitting data (DCH). The mobile station receives the notification CH at each Sub-frame, determines the guard interval, and performs DCH processing.
In addition, in Patent Reference 2, below, as shown in FIG. 5 as the Conventional Example 2 related to the determination of the guard interval length, a system wherein the mobile station detects the guard interval length from DCH information without using other information (blind detection) and performs DCH processing is disclosed.    [Patent Reference 1] Japanese Published Patent Application No. 2000-244441    [Patent Reference 2] Japanese Published Patent Application No. 2002-247005